pgsql: Back-patch bgworker API changes to 9.3.

Back-patch bgworker API changes to 9.3.

Commit 7f7485a0cde92aa4ba235a1ffe4dda0ca0b6cc9a made these changes
in master; per discussion, backport the API changes (but not the
functional changes), so that people don't get used to the 9.3 API
only to see it get broken in the next release. There are already
some people coding to the original 9.3 API, and this will cause
minor breakage, but there will be even more if we wait until next
year to roll out these changes.

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/026bc46da33ab6a6f720b0d0500e8a95d075ab92

Modified Files
--------------
contrib/worker_spi/worker_spi.c | 23 +++++++++++------------
doc/src/sgml/bgworker.sgml | 6 +++---
src/backend/postmaster/postmaster.c | 3 ---
src/include/postmaster/bgworker.h | 7 ++++---
4 files changed, 18 insertions(+), 21 deletions(-)